TPMS and other car systems like keyless entry problems can be difficult to diagnose as unliked wired systems that are visible, they transmit wirelessly and therefore require specialist tools to observe. This is because wireless systems transmit and receive radio waves.

In the instance of TPMS sensors, they transmit information at a very low intensity in all directions in waves that are absorbed by the vehicle and the surrounding environment. While the signals are encrypted, they are practically unreadable by anyone at over 100 feet.

Most TPMS sensors transmit at 315MHz or 413MHz and this can sometimes also present a challenge. Many consumer devices such as home automation, security products and even phone chargers can emit electromagnetic radiation that can interfere with one another.

TPMS Sensors Don’t Transmit All the Time

TPMS sensors, however, don’t transmit all the time since that would be a quick way of draining the battery. Instead, they will only transmit when they detect movement using a component called an accelerometer. If it doesn’t detect movement, then it will stop transmitting, but will transmit instantly should it detect a sudden loss in tyre pressure.

Some models of TPMS sensors will also transmit a signal to notify the vehicle’s owner that there is reduced battery voltage when goes below a certain limit. However, this can be inconsistent, particularly in wintry weather, but this can correct itself as the tyres warm up.

Times where a TPMS receives a signal is rare. It happens when a specialist TPMS tool activates the sensor, which is done by emitting a pulse at a specific frequency as specified in the manufacturer’s instructions.

It Takes More Than Once

When transmission fails from the TPMS sensor, you won’t see a notification on your dashboard straight away. As mentioned previously, other items in the car and from the home can interfere, but it could also be sensors from other vehicles transmitting at the same time, or the sensor being behind a brake calliper. It waits for several missed signals before reporting a problem.

You can measure and confirm a TPMS sensor’s transmission by searching for a result in the scan tool data PIDs. It’s possible it could be a command to unlick the doors that is seen in the keyless entry module or BCM. You can also measure the intensity of the signal to measure the signals coming from smart cards and key fobs.

Keyless Entry Systems

Carrier waves are used by keyless entry systems on frequencies between 125 kHz and 140 kHz. The waves have information between the keys held by the driver and the antennas in the vehicle, sent by both keys and antennas. The antennas of these systems are usually located in the doors, the central console, and the tailgate. For security, the signals have a very short range.

You won’t be able to decode the information, but specialist probes can be used to test antennas and smart keys. This can be useful when diagnosing problems where the vehicle owner is unable to open the door or start the vehicle.

Seven Tips for Diagnosing TPMS Signalling Problems

  1. Home appliances and security systems can be sources of interference, so carry out relearn of TPMS sensors away from them and other vehicles
  1. When a vehicle isn’t receiving signals from a sensor during the relearn procedure, you can try moving the vehicle a few feet forward, which will unblock the sensor
  1. By using a TPMS tool that can interface through a OBCII port, you can help to avoid radio interference issues
  1. In situations where you can’t activate the TPMS sensor with the correct tool, you can push in on the valve stem, which will rapidly delate the tyre. You can then hole the tool near the tyre to check if the tool received the ID of the sensor
  1. Tyre balancers and mounting machines shouldn’t be around when performing relearning procedures of programming TPMS sensors. They can cause sensors to transmit by deflating tyres rapidly or activate them when they rotate, thus turning the TPMS light on the dashboard
  1. On longer vehicles like pickup trucks and vans, it should be noted that the signals can often be strained and get worse when filled with metal products.
  1. Dash cams that are mounted to the windshield can cause interference and even block the TPMS antenna in some cases